1. Field of the Invention
The present invention relates generally to motor protectors for hermetic refrigeration compressors.
2. Description of the Related Art
Present hermetic compressor motor microprocessor protection devices monitor motor line voltages, line currents, and other inputs such as temperature and motor speed to determine the state of an operating compressor. Motor speed is sometimes used as an estimate of torque, but as seen in FIG. 4 in the normal operating range of torque, the motor speed can vary significantly depending upon the motor operating temperature. Software algorithms are used to determine whether the compressor is operating within acceptable electrical, load, and thermal limits.
The present invention involves a method and device for protecting a refrigeration compressor motor from operating outside of its torque or load operating limits by using the input power to the motor. Input power correlates strongly to motor torque especially in the normal operating range of the motor. An input power range is determined for the compressor motor by measuring the input power at the extreme operating limits of the compressor over the torque or load range of the compressor. The motor input power is either directly measured in hardware or calculated in software using other hardware measurements such as input voltage and current. The attained motor input power is used as an input to a software algorithm to determine if the compressor is operating within acceptable torque operating limits. If the measured input power is found to be outside the determined input power range indicating the compressor motor is operating outside of its acceptable torque limits, the compressor motor will be shut down keeping the compressor motor from operating outside its torque operating range. Loss of refrigerant can cause an under-torque condition and loss of the condenser fan can cause an over-torque condition.
The present invention provides a motor protector system for a compressor. The system comprises a motor, input power lines, and a motor protector connected to the input power lines to control the power to the motor. The motor protector includes a microprocessor, an input power measurement device, and a switch means for controlling the connection of the power lines to the motor. The input power measurement device includes a motor power input signal to the microprocessor. The microprocessor includes memory having instructions for storing an operating input power limit range based on the torque limits of the motor and for comparing the power input signal to the operating input power limit range of the motor. The microprocessor controls the switch means to open when the input power signal is outside of the operating input power limit range of the motor.
The present invention provides a second embodiment of a motor protector system for a compressor. The system comprises a motor, input power lines, and a motor protector connected to the input power lines to control the power to the motor. The motor protector includes a microprocessor, voltage measurement device, current measurement device, and a switch means for controlling the connection of the power lines to the motor. The input voltage measurement device includes a motor voltage input signal to the microprocessor. The input current measurement device includes a motor current input signal to the microprocessor. The microprocessor includes memory having instructions for storing an operating input power limit range based on the torque limits of the motor, for calculating a power input value by using the voltage input signal and the current input signal, and for comparing the power input value to the operating input power limit range of the motor. The microprocessor controls the switch means to open when the input power value is outside of the operating input power limit range of the motor.
The present invention further provides a method of providing torque protection to a compressor motor. The steps comprise providing a motor protector having a microprocessor with memory and an input power switch, storing an operating input power limit range based on the torque limits of the compressor motor in the memory of the microprocessor, measuring the input power to the compressor motor, providing the measured input power to the microprocessor, comparing the measured input power value to the operating input power limit range of the compressor motor, and opening the input power switch to remove power from the compressor motor when the measured input power value is outside of the operating input power limit range.